1. Field of the Invention
The present invention relates to solid oxide fuel cells which generate electric power by the use of a flat plate-shaped solid oxide fuel cell element and a process for producing the same.
2. Related Art Statement
Recently, fuel cells have been noted as power generating devices. The fuel cell is device capable of directly converting chemical energy possessed by fuel to electric energy. Since the fuel cell is free from limitation of Carnot's cycle, the cell is a very promising technique owing to its high energy conversion efficiency, wide latitude of fuels to be used (naphtha, natural gas, methanol, coal reformed gas, heavy oil and the like), less public nuisance, and high electric power generating efficiency without being affected by scale of installation.
Particularly, since the solid electrolyte fuel cell (hereinafter referred to as "SOFC") operates at high temperatures of 1,000.degree. C. or so, activity of electrodes is very high. Thus, no catalyst of a noble metal such as expensive platinum is necessary. In addition, since the SOFC has low polarization and relatively high output voltage, its energy conversion efficiency is conspicuously much higher than those of the other fuel cells. Furthermore, since their constituent materials are all solid, the SOFC is stable and has long service life.
Since such fuel cells can be constructed by solid structural materials, various types of cell structures have been proposed. So-called flat plate-shaped solid oxide fuel cells are structurally very promising among them because their electric power per unit volume can be easily increased.
With the flat plate-shaped SOFC and monolithic SOFC, however, there are problems in production techniques and the like as follows.
An ion-conductive film of zirconia has been formed on a flat plate-shaped porous support by means of chemical vapor deposition (CVD), physical vapor deposition (PVD) or electrochemical vapor deposition (EVD). However, apparatuses for use in CVD, PVD and EVD are generally expensive and are large in scale. Moreover, film forming speed is low so that these methods are low in productivity and not suitable for mass production and it is difficult to obtain films of wide areas. Furthermore, internal resistance in the cells is high owing to diffusion resistance of fuel gases passing through porous supports.
A method has been proposed to form corrugated green sheets of an ion-conductive film of zirconia, an air electrode film and a fuel electrode film, press contact these green sheets and sinter them. In this method, however, since green sheets of different materials are sintered together, it is difficult to adjust their shrinkages during firing. Further, when the green sheets are press contacted with one another, the green sheets are likely to be cracked or fractured. Moreover, insulating layers tend to be produced at interfaces between the ion-conductive film and the electrode films after they are sintered together.
In order to overcome these problems, Japanese Patent Application Laid-open No. 1-128,359 disclosed a method in which an ion-conductive plate of zirconia is formed by sintering, electrodes are formed on both surfaces thereof to form a flat plate-shaped SOFC element, and such SOFC elements and insulating spacers are alternately laminated to form a SOFC.
However, the inventors' investigations revealed that these methods involved the following problems.
That is, columnar conductors have to be used to electrically connect the flat plane-shaped SOFC elements in parallel. Each of the columnar conductors contacts each air electrode or each fuel electrode at one location. Therefore, since electric current flows to this contact location between the conductor from the entire surface of each air electrode or fuel electrode along and in parallel to the filmy air electrode or filmy fuel electrode, electric resistance and voltage loss are very large when considered with respect to the entire SOFC. Further, since the diametrically round sectional area of the columnar conductor cannot be increased beyond a given value from the standpoint of the structure of the SOFC, the cross sectional area of the conductor is relatively small. Owing to this, particularly when the conductor is long, voltage loss is considerably great. Further, since the flat plane-shaped SOFC elements are connected in parallel, voltage available is today about 1 V at the maximum even by increasing the number of the elements. Thus, the parallel connection type SOFC is not practical.